1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device that uses a thin-film semiconductor having crystallinity. In particular, the invention relates to manufacture of an insulated-gate thin-film transistor (TFT).
2. Description of the Related Art
In recent years, the techniques of forming a TFT using a crystalline silicon film (particularly a film made of a material called polysilicon film) on a substrate having an insulative surface have been developed. A TFT using such a material is advantageous over a TFT using an amorphous silicon film in being capable of high-speed operation.
For this reason, extensive studies are now being made on the monolithic panel in which a pixel matrix circuit and driver circuits are formed on the same substrate as well as the system-on-panel structure in which signal processing logic circuits (memories, amplifiers, a CPU, etc.) are additionally formed in an integral manner. For example, the driver circuits and the logic circuits are formed as a composite circuit in which a CMOS circuit (inverter circuit) that is a complementary combination of an n-type TFT and a p-type TFT is used as a basic circuit.
The TFT that constitutes such a variety of circuits is a switching element that is turned on when a particular voltage (called a threshold value or a threshold voltage) is applied to the gate electrode and that is rendered off when a voltage lower than the particular voltage is applied. Therefore, precise control of the threshold voltage is very important for correct operation of a circuit.
However, there may occur an event that the threshold voltage of a TFT shifts to the positive or negative side due to indefinite factors in a manufacturing process, such as influences of mobile ions that have been introduced by pollution and a difference in work function or interface charge in the vicinity of the gate of the TFT.
Such a shift of the threshold voltage causes adverse effects such as impairing the switching element function and increasing the power consumption. Although the pollution-induced factors can be removed by, for instance, improving the process, the factors caused by a work function difference or the like are determined by the materials used and hence are difficult to remove in some cases.
The channel doping has been proposed to remove the latter factors. The channel doping is a technique of controlling, i.e., intentionally shifting the threshold voltage by adding an impurity element (typically, P, As, or B) that imparts one conductivity type to at least the channel forming region of a TFT. To control the threshold voltage to a desired value, it is necessary to control the addition amount of the impurity element very precisely.
The impurity element may be added by mixing it into a gas for forming an amorphous silicon film or a crystalline silicon film or by performing ion implantation or the like after crystallization. Further, the impurity element may be added selectively, i.e., only to a portion, to become a channel forming region, of a crystalline silicon film that has been shaped into an island-like pattern.
After concentrated studies for obtaining superior TFT characteristics, the present inventors invented a crystalline silicon film having much superior crystallinity. Conditions necessary for forming this crystalline silicon film will be described below briefly.
First, an amorphous silicon film is formed on a highly heat resistant substrate (for instance, a quartz substrate) and then crystallized by utilizing the technique disclosed by the present inventors in Japanese Unexamined Patent Publication No. 7-130652, which is a technique of adding a catalyst element (typically nickel) for accelerating crystallization to an amorphous silicon film and then crystallizing it by a heat treatment. The disclosure thereof is incorporated herein by reference.
After a crystalline silicon film has been obtained, the catalyst element is gettered by performing a heat treatment in an atmosphere containing a halogen element. This gettering step utilizes the metal element gettering effect of the halogen element. To obtain a sufficient gettering effect of the halogen element, it is preferable that the heat treatment be performed at more than 700.degree. C.
In the gettering step, the catalyst element remaining in the crystalline silicon film is combined with, i.e., gettered by, the halogen element to become a volatile halide which escapes into the air. The catalyst element is thus removed. As a result of the catalyst element gettering step, the concentration of the catalyst element in the crystalline silicon film is reduced to less than 1.times.10.sup.17 atoms/cm.sup.3 (preferably less than the spin density). In this specification, the impurity concentration is defined as the minimum value of measurement values obtained by secondary ion mass spectroscopy analysis.
A crystalline silicon film that is formed in the above-described manner has a feature that it is a crystal structural body as a collection of a plurality of rod-like or flat-rod-like crystals and microscopically the growth directions of the respective rod-like crystals are aligned in a particular direction. Further, the crystallinity inside the crystals has been greatly improved by the heat treatment of the gettering step.
However, in experimentally producing various kinds of TFTs by using crystalline silicon films of the above kind, the present inventors have found that a serious problem occurs in applying the above-mentioned channel doping. This is an phenomenon that in removing a catalyst element in the gettering step, an impurity element (B, P, As, or the like) that has been added to the surface layer and its vicinity of the crystalline silicon film is exhausted. This phenomenon is reported in IBM Technical Disclosure Bulletin, Vol. 1, No. 5, 1973. The disclosure thereof is incorporated herein by reference.
Since the concentration of the impurity element in the region (in the vicinity of the surface of the active layer) where a channel is to be formed is much reduced, the intended effect of the channel doping is not obtained, which makes it impossible to control the threshold voltage precisely.